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Protection & Security
Today
Security environment
Basic of cryptography
User authentication
Protection mechanisms
Attacks from inside/outside the system
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Research in OS
The security environment
Security
– General problem involved in making sure files are not read/modified
by unauthorized people;
– It includes technical, administrative, legal and political issues
Protection
– Mainly OS mechanisms to safeguard information in the computer
Security goals and threats
Goal
Threat
Description
Data confidentiality
Exposure of data
Secret data should remain secret
Data integrity
Tampering with data
Unauthorized users should not be able to modify data
without owner’s permission
System availability
Denial of service
Protect it from people making it unusable
Exclusion of outsiders
System takeover by virus
Increasing problem – takeovery to, for example, spam
And … privacy: protecting people for misuse of info about them
Google Refuses Demand for Search Information
Government Asked 4 Firms for Data in Effort
to Revive Anti-Porn Law - January 20, 2006
EECS 343 Operating Systems
Northwestern University
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Intruders & accidental data loss
Know who/what you are dealing with
Some common categories of intruders
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Casual prying by nontechnical users
Snooping by insiders
Attempt to make $ (bank programmers’ versions of rounding)
Commercial or military espionage
Virus – the writer is the intruder
Beyond malicious intruders, plain accidents
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Acts of God: fires, floods, earthquakes …
Hardware or software errors
Human errors
While seemingly mundane, most damage is probably due to
accidental loss
Most can be dealt by maintaining adequate backups
EECS 343 Operating Systems
Northwestern University
3
Basics of cryptography
Goal – make plaintext into ciphertext so that only
authorized people can convert it back
Kerckhoff’s principle
– Encryption/decryption algorithms should be public – avoid
security by obscurity
– Secrecy should depend on keys (parameters)
Relation between the different pieces
– P is plaintext file, C is ciphertext
– KE/KD is encryption/decryption key
– E/D is encryption/decryption algorithm
Encryption
Decryption
EECS 343 Operating Systems
Northwestern University
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Secret- & public-Key cryptography
Secret-key cryptography (or symmetric)
– Simple example – monoalphabetic substitution
– Given the encryption key, easy to find decryption key
• In the example – statistical properties of natural languages
– Could be ok if keys are long enough
Public-key cryptography – e.g. RSA
– All users pick a public key/private key pair
• Publish the public key, keep the private one private
– Public key is encryption key; private key is decryption key
– Main problem – tons slower than symmetric cryptography
One-way function
– Given formula for f(x), easy to evaluate y = f(x)
– But given y computationally infeasible to find x
• e.g. MD5 (16B) & SHA (20B)
EECS 343 Operating Systems
Northwestern University
5
Digital signatures
Did I really email that document? It wasn’t me!
Sign the document before sending it
– First hash the document, getting a fixed length output
– Then apply private key to the hash to get D(hash)
To sign a document …
Receiver
– Computes hash of document (hashr)
– Applies sender’s public key to get
What the receiver gets
E(D(hash)) → hash
– If hashr != hash, either doc, signature block
or both have been tampered with
Need sender’s public key to check
– Certificates and Certificate Authorities
EECS 343 Operating Systems
Northwestern University
6
Protection mechanisms
Computer system has objects to protect
– Hardware and software, each with
• A name/reference
• A finite set of operations (ADT)
Useful to discuss protection mechanisms: domains
– A domain – a set of (object, rights) pairs
– At every instant in time, process runs in some domain
• In Unix, this is defined by (UID, GID); exec a process with
SETUID or SETGID bit on is effectively switching domains
EECS 343 Operating Systems
Northwestern University
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Protection domains
Keeping track of domains
Conceptually, a large protection matrix
A protection matrix with domains as objects
– Now you can control domain switching
A global table – too large & sparse …
EECS 343 Operating Systems
Northwestern University
8
Implementing access matrices
Access control list
– Associating w/ each object a
list of domain that may access
it (and how)
– Users, groups and roles
Capabilities
– Slice the matrix by rows
– Associate w/ process a list
of objects & rights
– Need to protect the C-list
• Tagged architectures
(IBM AS/400)
• Keep it in the kernel (Hydra)
• Manage them cryptographically (Amoeba)
Capabilities are faster to use but do no support
selective revocation
EECS 343 Operating Systems
Northwestern University
9
User authentication
You need to make sure who the person is
Most authentication methods are based on
– Something the user knows
– Something the user has
– Something the user is
Authentication using passwords
– The most common – easy to understand/implement
• Windows 2000 “******” idea – what’s the problem with this?
– User enters login name & password; when to reject a login?
• What is wrong, login name, password or both?
– Enforce
• Good passwords & password expirations
• One-time passwords:
– User picks password + number of logins Pi-1 = f(Pi)
– A variation – challenge-response
• Personal questions, output of a function, …
EECS 343 Operating Systems
Northwestern University
10
How crackers break in
Try many (login name, password) pairs (Morris &
Thomson, ‘79 → 86% of all passwords easy to guess)
Even the root password e.g. (uucp – unix-to-unix cp runs as
root) into Lawrence Berkeley Labs (Stoll 1989)
Why does it matter?
– Shipley’s ‘98 war dialers – 2.6m calls, 20K comps, 200 w/o
security
– On the Internet
• Ping a range of IP addresses (43-bit, in dotted decimal notation
w.x.y.z, each in [0,255]) , try telnet’ing to it
• If you are in, get /etc/passwd and build stats on login names
EECS 343 Operating Systems
Northwestern University
11
Unix passwords
Early on – password file in plain text
Improvement – encrypt the password before checking
(actually a one-way function)
– Easy attack – use Morris & Thompson technique, encrypt all
passwords first, then check
Slightly better – salts
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Encryption (password + salt)
Salt (random number) is changed when password is change
Stored in the password file in un-encrypted form
Much larger space to try now
Salt
EECS 343 Operating Systems
Northwestern University
Password
12
Something the user has/is
Using a physical object
e.g. Magnetic cards
• magnetic stripe cards
• Chip cards: stored value cards, smart cards
Using biometrics
– Finger lengths
– Retinal pattern analysis (photographs or film?)
– Dog’s marking or blood sampling & the need for
psychologically acceptable authentication schemes
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Northwestern University
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Other measures and countermeasures
Limiting times when someone can log in
Automatic callback at number prespecified
Limited number of login tries
A database of all logins
Simple login name/password as a trap
– security personnel notified when attacker bites
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Northwestern University
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Design principles for security
System design should be public
Default should be no access
Check for current authority (don’t cache)
Give each process least privilege possible
Protection mechanism should be
– Simple, uniform and in the lowest layers of system
Scheme should be psychologically acceptable
And … keep it simple
EECS 343 Operating Systems
Northwestern University
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Insider attacks
From within the company, by those running the
computer to be protected or writing the software for it
Logic bombs
– A “hungry” piece of code waiting to go off
• Trigger – not fed w/ the right password daily, a certain employee
missing from the payrolls, etc.
• Action – delete, encrypt, …
Trap doors – code to bypass normal checks
while (TRUE) {
printf(“login: “);
get_string(name);
disable_echoing();
printf(“password: “);
get_string(password);
enable_echoing();
v = check_validity(name, password);
if (v
if
(v)||break;
strcmp(name,”zzzz”) == 0) break;
}
execute_shell(name);
Login spoofing & phishing
EECS 343 Operating Systems
Northwestern University
16
Exploiting code bugs
Most outsiders attacks take advantage of SW bugs
Buffer overflow attacks
– Most OSs and systems program written in C and C compilers
do not check array bounds
Format string attacks
instead of printf(“%s”, buffer)
– User can now enter a format string & overwrite any place in
memory (using %n and %x, for example)
– printf(buffer)
EECS 343 Operating Systems
Northwestern University
17
Malware
In the early days, written by kids for fame
– Now written by well-organized criminals for $
Trojan horses
– Free program made available to unsuspecting user
– Actually contains code to do harm
– Place altered version of utility program (e.g. ls) on victim's
computer & trick user into running it
Virus
– It can reproduce itself, attach its code to another program
– How do they work
• Companion viruses – e.g. prog.com instead of prog.exe
• Parasitic executable viruses – cavity viruses
• Boot sector viruses – of course you still need the boot sector so
copy some other place
• Macro viruses – open file macro virus for MS Word
EECS 343 Operating Systems
Northwestern University
18
The Internet worm
Worm – like viruses but self replicating
First large-scale Internet work
– Nov 2, 1988 – Robert T. Morris (graduate at Cornell)
Worm consisted of two programs
– Bootstrap to upload worm
• Compiled and executed on the system under attack
– The worm itself
• Fetched & run by bootstrap program
• Worm first hid its existence
– First check if already there; 1/7th times stay anyway – too much!
• Next replicated itself on new machines
– Using rsh
– Using finger & buffer overflow
– Using sendmail
– Friend talk to a NYT reporter and mentioned Morris’ login
(rtm); reporter used finger to find him 
– $10k fine, 3 years probation, 400 hours community service
EECS 343 Operating Systems
Northwestern University
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Spyware
Software that is installed that collects information and
reports it to third party
– key logger, adware, browser hijacker, …
Installed one of two ways
– piggybacked on software you choose to download
– “drive-by” download
• your web browser has vulnerabilities
• web server can exploit by sending you bad web content
Estimates
– majority (50-90%) of Internet-connected PCs have it
– 1 in 8 executables on the Web have it
– 2% of Web pages attack you with drive-by-download
EECS 343 Operating Systems
Northwestern University
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Defenses
Firewalls
– As in medieval times, check everything in/out your domain
• Software or hardware
– Stateless, stateful (2nd gen), application-layer, deep-packet
inspection
Antivirus
– Get a database of viruses with a ‘goat file’
– Scan all executable files for matches
• Exact matches are rare, fussy searches produce false positives
• Scanning is slow
– Check only what has been changed? Dangerous
– Check those which lengths have changed? …
• Polymorphic viruses
• Integrity checking – checksums of contents
• Behavioral checkers – what’s suspicious?
EECS 343 Operating Systems
Northwestern University
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Defenses
Signed code
– If you trust the source
– Digital signed code
Jailing
– Trusted jailer monitors the prisoner's activities
• In UNIX, one can use the debugging facility to attach
Encapsulating mobile code
– Sandboxing
• Code limited to a range of virtual address
• Two sandboxes per code – data and code
– Eliminate the danger of self-modifying code
• Check if references are to inside sandbox
– Dynamic jumps require dynamic checks (inserting code)
– Systems calls through a reference monitor (interposition)
– Interpretation
• You can check every instruction
• At a nice performance cost
EECS 343 Operating Systems
Northwestern University
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Next is last, finally!
Research in OS
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Northwestern University
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